Contributed by the IC Engine Division of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received February 2, 2016; final manuscript received February 19, 2016; published online April 19, 2016. Editor: David Wisler.

Abstract

Multicycle large-eddy simulations (LES) of motored flow in an optical engine housed at the University of Michigan have been performed. The simulated flow field is compared against particle image velocimetry (PIV) data in several cutting planes. Circular statistical methods have been used to isolate the contributions to overall turbulent fluctuations from changes in flow direction or magnitude. High levels of turbulence, as indicated by high velocity root mean square (RMS) values, exist in relatively large regions of the combustion chamber. But the circular standard deviation (CSD), a measure of the variability in flow direction independent of velocity magnitude, is much more limited to specific regions or points, indicating that much of the turbulence is from variable flow magnitude rather than variable flow direction. Using the CSD is also a promising method to identify critical points, such as vortex centers or stagnation points, within the flow, which may prove useful for future engine designers.

Resolved TKE fraction as a function of crank angle. Plotted are the minimum, 5th, and 50th percentile. While isolated cells may have low resolved TKE fractions, these values are limited to a handful of cells, as shown by the 5th percentile, which never drops below a resolution measure of 0.83.

Port pressure comparisons: (a) intake and (b) exhaust. Mean pressure traces from multiple experiments are each plotted individually. LES 95% CIs are provided every four crank angles. LES pressure predictions match the experiments very well, particularly the pressure oscillation frequencies. The magnitude of the LES pressure fluctuations can be slightly greater than experiments, particularly during the valve-open portion of the engine cycle.

Cylinder pressure. Plots have been enlarged (a) near the peak pressure at TDC or (b) around the lower pressure during intake and exhaust for clarity. Simulations predict higher peak pressures due to the lack of a blow-by model in the simulations. Pressures during the intake and exhaust strokes are closely matched between the experiment and LES.

Mean velocity contours in the Y = 0 cutting plane 100 deg ATDCex: (a) experiment and (b) LES. The intake jet predictions are similar, though experiments show higher mean velocities surrounding the counter-rotating vortices on either side of the jet.

Mean velocity contours in the Y = 0 cutting plane 300 deg ATDCex: (a) experiment and (b) LES. The squished tumble vortex flow pattern in similar between the experiments and LES data, but the experiment shows high mean velocities near the chamber boundaries.

Mean velocity contours in the Z = −5 cutting plane 300 deg ATDCex: (a) experiment and (b) LES. There is a uniform flow pattern toward the upper right of the image, with evidence of a “wake” behind the spark plug ground strap.

Velocity RMS in the Y = 0 cutting plane 100 deg ATDCex: (a) experiment and (b) LES. LES RMS is almost zero in the intake jet core, while experiments measure a high variability there. Downstream the RMS contours are similar, though the magnitude is higher in the experiments.

Velocity RMS in the Y = 0 cutting plane 300 deg ATDCex: (a) experiment and (b) LES. Experiment and LES both show relatively uniform RMS in the middle of the cylinder, transitioning to lower values near the chamber surfaces. Experimental RMS is greater, but consistent with the higher mean velocities in this cutting plane.

Velocity RMS in the Z = −5 cutting plane 300 deg ATDCex: (a) experiment and (b) LES. Experiment and LES RMS magnitudes are approximately the same with similar distributions in the cutting plane. LES data show effects from the ground strap wake while the experiment does not.

CSD in the Y = 0 cutting plane 100 deg ATDCex: (a) experiment and (b) LES. Experimental data contain an extra region of high CSD in the upper left corner of the cutting plane not present in the LES, but the other features are matched well.

CSD in the Y = 0 cutting plane 300 deg ATDCex: (a) experiment and (b) LES. The LES results show high CSD at the tumble vortex core, with elevated values extending along the major axis of the squished vortex, also visible in the experimental results.

CSD in the Z = −5 cutting plane 300 deg ATDCex: (a) experiment and (b) LES. Experimental data show almost no variation in flow direction. LES shows slightly higher CSD next to the ground strap and in a region outside the viewable area of the experiment which corresponds to a region with high out-of-plane velocity components.

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